Field of the Invention
The present invention relates to a method of manufacturing a carcass structure for tires for vehicle wheels.
The invention also relates to a carcass structure for tires for vehicle wheels obtainable by the above method, said carcass structure comprising; at least one carcass ply formed of strip-like sections each of which extends in a substantially U-shaped conformation following the cross-section outline of the tire and comprises one or more thread-like elements longitudinally arranged parallelly of each other and preferably at least partly coated with a layer of raw elastomer material; and a pair of annular reinforcing structures each engaged close to a respective inner circumferential edge of the carcass ply.
Manufacture of tires for vehicle wheels involves formation of a carcass structure essentially made up of one or more carcass plies substantially having a toroidal conformation and presenting their axially opposite side edges engaged to respective annular, circumferentially inextensible, reinforcing elements usually referred to as xe2x80x9cbead coresxe2x80x9d.
Applied to the carcass structure, at a circumferentially outer position, is a belt structure comprising one or more belt strips having the shape of a closed ring, which are essentially made up of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
Then, at a circumferentially outer position of the belt structure a tread band is applied which usually consists of a strip of elastomer material of suitable thickness.
It is to point out that, to the aims of the present description, by the term xe2x80x9celastomer materialxe2x80x9d it is intended a rubber blend in its entirety, i.e. the assembly formed of a base polymer suitably amalgamated with mineral fillers and/or process additives of various types.
Ultimately, a pair of sidewalls is applied to the opposite sides of a tire being manufactured, each of said sidewalls covering a side portion of the tire included between a so-called shoulder region, arranged close to the corresponding side edge of the tread band, and a so-called bead arranged at the corresponding bead core.
The traditional production methods essentially provide that the above listed tire components should be first made separately of each other, to be then assembled during a manufacturing step of the tire.
For example, for making the carcass ply or plies to be associated with the bead cores in order to form the carcass structure, it is first required that, by an extrusion and/or calendering process, a rubberized fabric comprising longitudinally-disposed continuous textile or metal cords should be produced. This rubberized fabric is submitted to a transverse cutting operation to produce sections of predetermined lengths that are subsequently joined together so as to give origin to a continuous ribbon-like semifinished product, having transversely-arranged parallel cords.
This manufactured article must then be cut into sections the length of which is correlated with the circumferential extension of the carcass to be produced.
Manufacturing methods have been also proposed in which, instead of resorting to production of semifinished articles, the carcass structure is made directly during the tire manufacturing steps.
For example, U.S. Pat. No. 5,453,140 herein referred to as an example of the most pertinent state of the art, discloses a method and an apparatus forming a carcass ply starting from a single cord that has been previously wound up on a reel.
In accordance with the method and apparatus described in the above patent, at each working cycle of the apparatus the cord taken up from the reel by power-driven pulling rollers and maintained taut by a pneumatic tensioning system is cut to size so as to obtain a section of a predetermined length.
The cord section is taken up by a grip element mounted on a belt looped around power-driven pulleys to be transversely laid down on the outer surface of a toroidal support.
The section ends are then engaged by folding members of the belt type operating on opposite sides of the toroidal support to radially apply the cord section to the toroidal support itself by slider elements acting like fingers along the section side portions.
Repetition of the above described working cycle leads to deposition of these cord sections in a circumferential side-by-side relationship until the whole circumferential extension of the toroidal support is covered.
Necessarily, the toroidal support is previously coated with one or more raw rubber layers having a dual function, that of adhering to the cords laid down thereon so as to conveniently hold them in a fixed positioning, and that of forming an inner air-proof layer in the finished tire.
Tires obtained by this manufacturing method have a carcass structure in which the carcass ply or plies are formed of individual cords each having two side portions axially spaced apart from each other and radially oriented relative to the rotation axis of the tire, and a crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tire beads, the opposite ends of the individual cords forming a carcass ply are arranged in an alternating sequence, at axially opposite positions relative to an annular anchoring element forming said bead core, having the shape of a crown made up of radially-overlapping thread or wire coils, as can be viewed from Patent EP 0 664 231 and U.S. Pat. No. 5,702,548.
In the above mentioned art all cords forming the carcass ply or plies however, are substantially arranged in the neutral axis of resistance to bending of the respective bead. Under this circumstance, the structural resistance of the beads must necessarily rely on the stiffness of the filling inserts of very hard elastomer material incorporated into the bead structure, the behaviour of which feels the effects of temperature changes due both to environmental factors and to stresses produced during normal operation.
In Patent FR 384 231 it is proposed to make a carcass structure by deposition on a toroidal support, of a series of rectangular small bands of rubberized fabric disposed circumferentially in side-by-side relationship and arranged in radial planes relative to the geometric axis of the support drum itself. Deposition of the small bands is carried out in such a manner that the end flaps of two non-consecutive small bands are partly covered with the end flaps of the small band interposed therebetween. Spaces existing between the end flaps of the covered small bands are filled with trapezoidal inserts applied to the end flaps of the small band placed in overlapping relationship thereon. Deposition of the small bands is executed in different overlapped layers, the number of which is correlated with the thickness to be given to the carcass structure. The presence of said trapezoidal inserts gives rise to thickening of the carcass structure at the bead regions, where it has a thickness which is twice that found at the crown.
In U.S. Pat. No. 4,248,287 it is disclosed a method according to which formation of the carcass structure involves that a plurality of layers each formed of radial strips consisting of rubberized threads and circumferentially disposed in side by side relationship should be laid down on a toroidal drum. When deposition has been completed, at the bead region two bead cores are applied and the end flaps of the carcass layers formed by the radial strips are then turned back around them.
The Applicant has found that important advantages can be achieved, both in terms of simplification of the production processes and in terms of improvement of the behavioural features of the tire, if the carcass ply or plies are manufactured by conveniently laying down strip-like sections, each comprising a plurality of cords parallel to each other incorporated into an elastomer layer, onto a rigid toroidal support.
In this connection the Applicant has already developed several manufacturing methods being the object of respective European patent applications.
For instance, in European patent applications No. 97830731.2 and No. 97830733.8 a manufacturing method and a tire are respectively described in which the carcass structure is obtained by making a first and a second carcass plies each accomplished by strip-like sections laid down sequentially in a circumferential side-by-side relationship.
Tires obtained as described in these patent applications have the end portions of the strip-like sections belonging to the first and second carcass plies disposed on respectively opposite sides relative to the annular reinforcing structures of the beads.
This expedient, in combination with the respectively crossed orientation of the strip-like sections belonging to one and the other carcass plies, offers important advantages in terms of structural resistance of the tire close to the beads and the sidewalls.
In the European patent application No. 98830472.1, in the name of the same Applicant as well, accomplishment of a carcass ply is proposed which is carried out by depositing a first and a second series of strip-like sections in alternated sequence, in which the sections belonging to the first and second series terminate at respectively opposite sides relative to the reinforcing structures of the beads.
Advantages can be thus achieved in terms of structural resistance at the tire beads and sidewalls even in the presence of a single carcass ply.
In accordance with the present invention, it has been found that, for achieving exceptional qualities of mechanical resistance to stresses induced in the acceleration and deceleration steps, as well as in a condition of slip running, it is convenient that two carcass plies should be arranged which are each formed of two series of strip-like sections laid down in an alternated sequence, the sections of each ply terminating at respectively opposite sides of a respective portion of the reinforcing structure at the bead.
In particular, it is an object of the invention to provide a method of manufacturing a carcass structure for motor-vehicle tires, characterized in that it comprises the steps of: preparing strip-like sections each comprising longitudinal and parallel thread-like elements coated at least partly with at least one layer of raw elastomer material; laying down a first series of said strip-like sections onto a toroidal support, each of them extending in a substantially U-shaped configuration around the cross-section outline of the toroidal support itself and circumferentially distributed according to a circumferential pitch corresponding to a multiple of the strip-like section width; applying first primary portions of annular reinforcing structures against end flaps of said strip-like sections belonging to the first series, at axially opposite positions relative to an equatorial plane of the support drum; laying down on the toroidal support, at least one second series of said strip-like sections each extending according to a U-shaped conformation around the cross-section outline of the toroidal support, between two consecutive sections of the first series, in order to define a first carcass ply together with said last-mentioned sections, each of the sections of the second series having end flaps overlapping the respective primary portions of the annular reinforcing structures at an axially opposite position relative to the end flaps of the sections of the first series; laying down on the toroidal support, a third series of said strip-like sections each extending according to a substantially U-shaped conformation around the cross-section outline of the toroidal support itself and circumferentially distributed according to a circumferential pitch corresponding to a multiple of the strip-like section width; applying second primary portions of said annular reinforcing structures against the end flaps of said strip-like sections belonging to the third series, at axially opposite positions relative to the first primary portions; laying down on the toroidal support, at least one fourth series of said strip-like sections each extending according to a U-shaped conformation around the cross-section outline of the toroidal support, between two consecutive sections of the third series, in order to define, together with said last-mentioned sections, a second carcass ply overlapping the first carcass-ply, each of the sections of the fourth series having end flaps overlapping the respective second primary portions of the annular reinforcing structures at an axially opposite position relative to the end flaps of the sections of the third series.
In more detail, the strip-like sections making up the first and second carcass plies respectively, are provided to be laid down in a respectively crossed orientation, preferably in an orientation inclined at an angle included between 15xc2x0 and 35xc2x0 relative to a circumferential extension direction of the toroidal support.
According to a preferential embodiment of the invention, also carried out is the step of applying additional portions of the annular reinforcing structures against the end flaps of the strip-like sections belonging to the fourth series, so that each of said end flaps is enclosed between the second primary portion and the additional portion of the respective annular reinforcing structure.
Advantageously, each of said strip-like sections is laid down so as to form two side portions substantially extending in the direction of the geometric rotation axis of the toroidal support at mutually spaced apart positions in an axial direction, and a crown portion extending at a radially outer position between the side portions, the crown portions of the strip-like sections belonging to the first and second carcass plies respectively being disposed consecutively in side by side relationship along the circumferential extension of the toroidal support.
In more detail, the side portions of said strip-like sections are made radially converge towards the geometric rotation axis of the toroidal support.
It is also provided that the side portions of each strip-like section belonging to the first series and the third series respectively should be each partly covered with a side portion of at least one circumferentially consecutive section belonging to the second series and the fourth series respectively, at a stretch included between a radially outer edge of the respective primary portion of the annular reinforcing structure and a transition region between said side portions and crown portions.
In accordance with a further aspect of the invention, to be also adopted independently of that which has been previously pointed out, accomplishment of at least one of said first and second primary portions of each annular reinforcing structure comprises the steps of: laying down at least one elongated element in concentric coils to form a circumferentially inextensible annular insert substantially in the form of a crown; forming at least one filling body of raw elastomer material; joining the filling body to the first circumferentially inextensible annular insert.
Preferably, said elongated element is deposited directly against the end flaps of the strip-like sections previously deposited on the toroidal support, to form said first annular insert directly in contact with the strip-like sections themselves.
According to a possible preferential solution, said filling body is formed by depositing a continuous strip of elastomer material directly against the annular insert previously applied to the end flaps Of the strip-like sections previously laid-down.
Alternatively, the elongated element is laid down in a forming seat defined in a moulding cavity in which the filling body is subsequently formed, so that joining of said filling body to the annular insert is carried out concurrently with formation of the filling body itself.
In more detail, said joining step can be carried out by applying the filling body against said annular insert previously applied to the end flaps of the strip-like sections laid down on the toroidal support.
Preferably, formation of said additional portion of each annular reinforcing structure comprises the step of depositing at least one elongated element in concentric coils to form an additional circumferentially inextensible annular insert substantially in the form of a crown.
In more detail, said elongated element is preferentially laid down directly against the carcass structure during the formation step on the toroidal support.
It is a further object of the invention to provide a carcass structure for vehicle wheel tires, characterized in that said first carcass ply comprises: a first and a second series of strip-like sections arranged in a mutually alternating sequence along the circumferential extension of the carcass structure, said second carcass ply comprises a third and a fourth series of strip-like sections arranged in a mutually alternating sequence along the circumferential extension of the carcass structure, each of said annular reinforcing structures comprising at least: one first primary portion having an axially inner side turned towards end flaps of the sections belonging to the first series and an axially outer side turned towards end flaps of the sections belonging to the second series, and one second primary portion having an axially inner side turned towards end flaps of the sections belonging to the third series and an axially outer side turned towards end flaps of the sections belonging to the fourth series.
In more detail, sections of the first and second series extend in a crossed orientation with respect to the strip-like sections of the third and fourth series, preferably at an angle included between 15xc2x0 and 35xc2x0 relative to a circumferential-extension direction.
It is preferably provided that each of said annular reinforcing structures further comprises at least one additional portion disposed against the end flaps of the strip-like sections belonging to the fourth series, on the opposite side relative to the second primary portion of the annular structure itself.
Advantageously, each of said strip-like sections has two side portions substantially extending towards a geometric axis of said carcass structure at mutually spaced apart positions in an axial direction, and a crown portion extending at a radially outer position between the side portions, the crown portions belonging to the sections of the first and second series respectively, and of the third and fourth series respectively, being arranged in mutual side by side relationship along the circumferential extension of the carcass structure.
It is provided that the side portions of said strip-like sections radially converge towards a geometric rotation axis of the carcass structure.
In more detail, the side portions of each strip-like section belonging to the first and the third series respectively should be each partly covered with a side portion of at least one adjacent strip-like section belonging to the second and fourth series respectively, at a stretch included between a radially outer edge of the respective primary portion of the annular reinforcing structure and a transition region between said side portions and crown portions.
Advantageously, the individual strip-like sections belonging to one of said series respectively, are disposed according to a circumferential distribution pitch corresponding to a multiple of the width of the strip-like sections themselves.
Preferably, the strip-like sections each have a width included between 3 and 15 mm, and each of them comprises three to eight thread-like elements.
It is also preferably provided that said thread-like elements should be disposed in the respective strip-like sections according to a mutual distance between centres not lower than 1.5 times the diameter of the thread-like elements themselves.
In accordance with a further independent aspect of the invention, each of said first and second primary portions of each of said inextensible annular structures comprises: a circumferentially inextensible annular insert substantially in the form of a crown disposed coaxially with the carcass structure and close to an inner circumferential edge of the carcass plies, said annular insert being made up of at least one elongated element extending in concentric coils; a filling body of elastomer material having one side joined to the annular anchoring insert.
Preferably, each of said annular reinforcing structures further comprises at least one additional portion disposed against the end flaps of the strip-like sections belonging to the fourth series, on the opposite side relative to the second primary portion of the annular structure itself.
Advantageously, said additional portion comprises an additional circumferentially-inextensible annular insert substantially in the form of a crown, made up of at least one elongated element extending in concentric coils and disposed coaxially with the carcass structure at a position axially close to the filling body of the second primary portion of the respective inextensible annular structure. Preferably, the filling body of elastomer material of each of said primary portions has a hardness included between 48xc2x0 and 55xc2x0 Shore D at 23xc2x0 C.